Introduction
Most studies of health of mobile phone users have been case-control studies of brain tumors. With only a few exceptions, exposure estimates in these studies have been based on self-reports of mobile phone use. Because self-reports are prone to recall bias (exposure information affected by disease status) and random errors, the accuracy of such estimates is questionable. Another potential source of error in case-control studies is selection bias resulted from low participation of control subjects who do not use mobile phones. A prospective cohort study using data from mobile network operators on the number and duration of calls is an approach that not only avoids these problems, but also allows studying multiple health outcomes. Exposure assessment in such studies can be supplemented by questionnaire data on the mode and circumstances of use.

Objective
The overall objective of this study was to evaluate feasibility of a prospective cohort study in Finland. Specific objectives were to evaluate the agreement between self-reported and operator-derived estimates of call time, consistency of mobile phone use over time, and to compare alternative approaches to improve participation in a cohort study.

Methods
Random samples of eligible subjects were selected from databases of two major mobile network operators. A total of 5,400 subjects were identified. Two approaches for mailing the study materials were used. Some subjects were sent an invitation letter and an informed consent form in the first mailing; the study questionnaire was sent only after the subjects signed and returned the consent form. Other subjects received all the materials in one mailing. Two versions of the questionnaire, long and short, were used. One third of the subjects received the shorter questionnaire. The network operators provided mobile phone use data for a three-month period in three consecutive years.

Results and Interpretation
The overall participation rate was 12%. Type of questionnaire or mailing approach had no effect on participation rate. The participation rate was highest (15%) among the oldest people (60-69 years) and lowest (9%) among 40-49-year old subjects. It was slightly higher in women (13%) than in men (11%) and increased with increasing level of mobile phone use from 9% among light users (<30 min per month) to 12% among intermediate users (30-359 min per month) and 14% among heavy users (≥360 min per month). The operator data on mobile phone use were available for 83% of the participants. Self-reported and operator-derived data were moderately correlated (Spearman’s rank correlation coefficient 0.6, 95% CI 0.54-0.66 for monthly call times). Self-reports overestimated mobile phone use in comparison with data from mobile network operators. The median ratio between self-reported and operator-derived data was 2.2 for call duration and 1.6 for number of calls. The discrepancy increased with increasing level of mobile phone use. The monthly call time and the number of calls were strongly correlated between the consecutive years.

Conclusion
The authors concluded: “The agreement between self-reported mobile phone use and operator databases was moderate and overestimation of the call time by participants was common. A prospective cohort study would be feasible in Finland, although the potentially low participation rate would increase the resources required for recruitment”.